The invention concerns a pressure bar for a belt grinding machine including a longitudinally extending carrier with a base plate facing the grinding belt having a plurality of bores arranged in a matrix, in which bores bolt-shaped pressure members are held for free axial movement, which bolts are intended to engage the grinding belt with their free ends and are urged in the direction toward the grinding belt by a hose inflatable with a pressure fluid and located on the side of the base plate facing away from the grinding belt.
In the deburring and descaling of large sheet metal parts by means of a belt grinding machine different problems arise. The sheet metal parts to be worked in general are not even. The larger the parts the larger in general are the tolerances to be evened out by the grinding belt or pressure bar in order to reach a uniformly flat finish or to grind off from the sheet the burrs whose removal is desired. Work pieces of large measurement also require large grinding performances. Moreover because of their large dimensions the parts are so heavy that their handling is extremely difficult. When contoured pieces are cut out of the sheets by automatic cutting, plasma cutting or similar methods melted metal runs downwardly and forms a burr on the underside of the sheet which is ground off. In order not to have to turn the parts coming out of the cutting apparatus, it has already been proposed in German Utility Model G 87 07 974 to grind the sheet metal part from below. This answers the difficulty in the handling of large work pieces.
In wide belt grinding machines which are also employed in the solution described in German Utility Model G 87 07 974 the grinding belt length between the two guide rolls is relatively short. A small displacement of the grinding belt on the short stretch between the guide rolls creates therefore a large reaction force so that a smoothing out of tolerances is possible only in relatively small areas. A considerable improvement has already been achieved here by a pressure bar as described in European Patent Application 0,210,654.
Wide belt grinding machines also have the disadvantage arising from their construction, that the spacing between the guide rolls cannot be diminished below a certain amount because of the pressure bar lying between them. This again means that the minimum length measured in the work piece feed direction and the minimum thickness of the work pieces to be worked in a wide belt grinding machine must be larger than for work pieces worked in a transverse belt grinding machine in order that the work pieces can pass problem free through the wide belt grinding machine, since in the grinding zone they can only be held on one side.
This difficulty can be solved with a grinding belt which moves in a direction perpendicular to the feed direction of the work pieces and which in the feed direction of the work pieces has only a small width. Transverse belt grinding machines for grinding wood have been known for a long time, in the case of which numerous solutions for controllable pressure bars have been proposed wherein the grinding belt is pressed against a work piece in accordance with the shape of the work piece. In the attempt to substitute one such transverse belt grinding machine for the grinding of metallic work pieces it has been shown that neither the grinding belts nor the sliding coating arranged between the pressure bar and the grinding belt stand up to the demands of the grinding process. The belts and the sliding coating after a short time are permanently thermally damaged.
The invention has as its object the provision of a pressure bar of the previously mentioned type which makes possible the grinding, especially for deburring and descaling, of large metallic work pieces with a transverse belt grinding machine.
This object is solved in accordance with the invention in that the previously mentioned hose of the pressure bar is surrounded by a plurality of frame-shaped pressure shoes which in the longitudinal direction of the carrier are arranged next to one another and are each adjustable perpendicularly to the base plate by a positioning apparatus, in that each shoe has bores in the side of the shoe facing the base plate registering with the bores of the base plate for receiving the pressure members, and in that inside of each pressure shoe between the hose and the adjacent ends of the pressure members is a flexible pressure distributing element having a higher stiffness than the material of the hose.
With the presently described pressure bar trouble free grinding results can be achieved with a transverse belt grinding machine with little wear of the grinding belt. With the pressure bar of the invention it is possible to bring the pressure members and therewith the grinding belt into engagement with the work piece only in certain areas determined by the shape of the work piece and of the problem areas. Therefore, the engagement length of the grinding belt with the work piece is reduced to a minimum as a result of which the heating of the grinding belt is substantially reduced. The comparatively soft hose passing through the shoes permits relative movement between the individual shoes. The stiff pressure equalizing element in each pressure shoe serves to provide a flat pressing of the individual pressure members inside a pressure shoe, but permits at the same time a relative movement between the individual pressure members inside of the same shoe so that not only can large tolerances be evened out, but also burrs and flat areas of the work piece can be ground which are smaller than the pressure face of one work shoe.
Preferably the pressure distribution element is formed by a sleeve inside each pressure shoe surrounding a section of the hose. In this way the pressure distribution element can be formed from a section of another hose. One such sleeve is simple to make and assures a good seating on the first hose.
Preferably the pressure shoes and their associated positioning devices, which for example can be pneumatic or hydraulically actuated work cylinders, are so moveably connected that the pressure shoes are each universally pivotal and rotatable to the degree permitted by the play of the pressure members in their bores. Therefore, no particular guides are necessary for the pressure shoes.
Preferably the bolt-shaped pressure members are secured against falling out of the bores of the base plate and to the pressure shoes by stops on their end portions. In connection with this each bolt-shaped pressure member preferably has a head on its side facing the grinding belt, which has the additional advantage that the specific loading of the grinding belt is reduced. At the other end it is sufficient if the bolt is secured by a snap ring or the like. To facilitate the moveability of the pressure shoes relative to one another, in accordance with the invention the pressure shoes are so formed that the remote horizontal frame side of each pressure shoe, which is spaced from the base plate and connected to the positioning device, in going from each vertical side surface of the frame to the middle of the shoe decreases in size.
To avoid areas in which no pressure is applied to the work piece and in which therefore no grinding takes place, which areas exist in a matrix type arrangement of the pressure members, the bores of the base plate and of the horizontal frame side of the pressure shoe facing the base plate are displaced relative to one another in the direction of the hole columns and if necessary also in the direction of the hole rows. The displacement is so chosen that the pressure members of one column or row are displaced only a fraction of the bolt diameter with respect to the pressure members of the neighboring column or row, and for a given bolt diameter and bolt spacings the amount of the displacement and the number of the pressure member columns or rows are so chosen that in each plane parallel to the feed direction of the work pieces and perpendicular to the grinding plane the grinding length is at least nearly the same.
The invention further concerns a belt grinding machine for grinding flat work pieces in a pass-through method, including a work piece support and at least one grinding assembly with an endless grinding belt circulating transversely to the feed direction of the work pieces, and a pressure bar for pressing the belt to the upper surface of the work pieces which bar is made in the above-described way. The grinding assembly can be arranged above the work piece support. For reasons of easier handling of the work pieces the grinding assembly is, however, preferably arranged below the work piece support. In both cases it is generally sufficient that the pressure members be actively adjustable only in the direction toward the work pieces, since the pressure members in other respects are rearwardly displaced from their initial positions spaced from the work piece by the belt tension (grinding assembly above the work piece support) or by the force of gravity (grinding assembly below the work piece support).
To reduce the thermal loading of the grinding belt, at least two or perhaps more grinding assemblies can be arranged behind one another in the feed direction of the work pieces through the belt grinding machine, with the length and position of the pressure bar of the grinding assembly being so chosen that the resulting grinding zones collectively cover at least the entire width of the transport path. This reduces for each individual grinding belt the relationship of the engagement area to the belt length and therefore also reduces the thermal loading. Alternatively, several grinding assemblies can be arranged behind one another whose grinding zones each extend over the full width of the transport path; the cutting performance of each assembly however being reduced to reduce the thermal loading.
Further features and advantages of the invention are apparent from the following description, which in conjunction with the accompanying drawings explain the invention in connection with exemplary embodiments thereof.